Fuel cells (not fuel storage cells) use hydrogen gas to generate electricity. Several general types of fuel cells exist. The present invention is concerned with PEM (proton exchange membrane) fuel cells.
A PEM fuel cell is typically comprised of a unit cell, shown “blown apart” in FIG. 1. The components of a PEM fuel cell include the hydrogen side bipolar plate 10, two sealing gaskets 12, a hydrogen flow field 14, a membrane electrode assembly (MEA) 16, an air flow field 18, and an air side bipolar plate 20. FIG. 2 depicts this PEM fuel cell assembled. A number of these PEM fuel cells may be stacked together as shown in FIG. 3.
A key component of the PEM fuel cell is the membrane electrode assembly (MEA). Typically, MEAs consist of a PEM sheet coated on both sides with an “electrode assembly”. This electrode assembly coating is primarily a platinum metal (catalyst) supported on activated carbon. The electrode assembly is bonded to the PEM material. In some designs, the electrode assembly may be applied to the air and hydrogen flow fields that lay on either side of the MEA.
The anode side of the fuel cell typically contains hydrogen gas. Fuel cells can use either pure hydrogen or a reformate gas stream of hydrogen gas and carbon dioxide that also contains other gases that are byproducts of incomplete reformation. The platinum in the MEA disassociates the hydrogen molecule into atomic hydrogen (protons), which then attach to a platinum atom. Some of these protons are in contact with the PEM material and move into the PEM material. A preferred PEM material manufactured by DuPont is called NAFION®, a perfluorosolfonic acid/PTFE copolymer.
Due to energetic differences, the proton moves through the PEM in hydronium ions (H3O+) to the cathode or air side. The platinum on the cathode side of the MEA facilitates the combination of the proton with available oxygen, forming water. This electrochemical process needs the electron released when the hydrogen molecule is catalytically split. This electron moves from the catalytic site on the anode side through the hydrogen flow field (a conductive open mesh of carbon cloth or paper for example) to the anode side of the bipolar plate (typically solid carbon), to the cathode side of the bipolar plate, through the cathode flow field (similar to the anode flow field) to the cathode platinum, thus supplying the necessary electron. Note that throughout these discussions, the terms anode side and hydrogen side are used interchangeably and the terms cathode side, air side and oxygen side are used interchangeably. These PEM cells are stacked electrically in series as shown in FIG. 3, and the end cells are connected through an electrical load through which the electrons move to complete the circuit.
The hydrogen gas volume is sealed by the gaskets. The flow field and the hydrogen flow channels cut in the bipolar plate facilitate transport of the hydrogen gas to the MEA. The hydrogen must be pressurized to supply the gas to the MEA. Typically, this pressure is just a few pounds per square inch gauge. Some designs increase the pressure to several atmospheres, but the hydrogen is so fluid that such higher pressures do not add much net power. As mentioned above, some designs use reformate hydrogen from the conversion of fossil fuels to hydrogen, carbon dioxide, and trace gases from incomplete conversion to the primary gases.
The cathode side is constructed in the same way as the anode side with the cathode side being either air or pure oxygen. The latter is more efficient but the oxygen must be supplied and the materials compatibility is more difficult. Pressurization of the air to several atmospheres can boost the power output by 10 to 30%.
A bipolar plate connects the cathode of one cell to the anode of the adjacent cell with the consequence that the cells must be stacked one on the other to provide electrical continuity. This design approach forces an assembly approach called the fuel cell stack, which inhibits design flexibility and manufacturing cost.
The bipolar plates are typically constructed of carbon and can be molded or machined. Carbon has been the material of choice. Metal plates have been investigated for cost reduction. The operational concern with metal plates is that metallic ions will be carried into the PEM material and tie up polymer strings thereby reducing the hydrogen transport efficiency. The presence of standing water with significant voltages that would be found by using cells in series with common flow channels aggravates this potential problem. Metal plates appear to be used in several commercially available fuel cells.
Management of the humidification of the PEM material is critical to efficient cell operation. As mentioned above, proton transport is via hydronium ions. The presence of the latter increases with the amount of water content in the PEM material. The resistance of a cell to hydrogen transport is easily measured and can change a couple of orders of magnitude due to the hydration of the PEM material. Therefore, one of the key cell design elements is the hydration of the PEM material. Often designers add water to the air and hydrogen sides to help hydrate the PEM material. The proton transport of the hydronium ion moves water from the anode side to the cathode side of the MEA. Additionally, product water is formed on the cathode side in completing the electrical circuit. The presence of product water and hydrated cathode air can create a water vapor partial pressure to push water back to the anode side. If the cathode air is too dry, the product water will preferentially move into the air. Moisturizing the hydrogen gas aids in humidifying the PEM membrane.
Heat management is also of concern. Fuel cells are about 40 to 60% efficient. The residual heat is primarily generated by the cathode catalytic operation. About 25 to 30 percent of this waste heat is removed by evaporation of the product water. The remaining heat must be actively removed. Attempts to remove this heat by increasing the cathode airflow will reduce the humidity of the air stream and the PEM material will tend to dry out. Consequentially, only a small part of the heat can be optimally removed via a cathode air stream. Another approach is to over humidify the anode side and force excess water through the PEM material. This water then evaporates cooling the cell. Finally, higher temperature operation facilitates heat removal and increases the amount of water the PEM material can hold, but these higher temperatures tend to dry out the PEM material. Some designs put cooling plates between the bipolar plates to help remove the internal heat. Some design approaches put cooling plates between each cell and some do not. Also, liquid coolant channels can be put into the bipolar plates, and the heat is transferred to a separate coolant/environment heat exchanger.
Some of the problems of this typical design approach are:
The bipolar plates are expensive to make. Considerable efforts by others have been made to cast the carbon or to use metal bipolar plates. The latter have been plagued with contamination and that contamination is aggravated by the complex flow pattern through the bipolar plates, which naturally traps liquid water.
The airflow through the bipolar plates is tortuous and creates unnecessary pressure losses.
The airflow is from one side to another, consequentially creating a temperature gradient across the PEM material that results in less efficient operation.
The large reliefs in the plates to accommodate gas flow create an inefficient electrical, electrochemical geometry. The pressure on the pads compresses the flow field, which restricts gas penetration but enhances electrical contact. In the channels, the opposite is true, especially for the flow of electrons that make contact only laterally through the flow field. This gross inhomogeneity leads to lower efficiencies.
The thermal management concept of removing significant heat by forced convection from the cathode side is inefficient. The approach of removing heat from less than 100% of the cells is inefficient. The approach of cooling the plates with a coolant and transporting that coolant to a secondary heat exchanger is inefficient. The approach of using a heat conduction plate to remove the heat from the cells to the sides of the stack for cooling is inefficient, creates temperature gradients in the stack, and provides a relatively small cooling fin structure.
The non-uniformity of the bipolar plate pattern creates non-uniformity in the humidification of the PEM material.
The hydrogen side is often more pressurized than the air side. The pattern of the channels of the bipolar plates ensures that the PEM material is not 100% supported and excess hydrogen pressure can rupture the PEM material. Likewise, if the cell is operated after the hydrogen supply is turned off, a vacuum can be created on the hydrogen side and the PEM material can rupture.
The overall design approach results in piece part assemblies and ancillary equipment of some complexity to move the air through the cathode side, humidify the supply gases, and mange the humidification of the PEM material. This design approach is also thermally and electrically inefficient. Furthermore, the bulkiness of the design and the decoupling of humidification (injection of water for example) and cooling (remote heat exchanger) leads to a design that is slow to respond to electrical load and environmental changes.
The stack design with such massive carbon bipolar plates forms a large thermal mass that results in a large thermal time constant. Large thermal time constants extend startup time and reduce response to load change.
Accordingly, there is a need for a PEM fuel cell with improved efficiency, improved airflow, improved thermal management, and reduced cost. The present invention satisfies these needs and provides other related advantages.